Hi, I am having a linux user space application which gets data from an interface (eg. eth0) using a packet socket. The application has a fast path and a slow path. In the fast path the packets are processed by the application and sent out via the packet socket. Certain packets need processing by linux IP stack -- this constitutes the slow path. I use a tun/tap interface to inject the packet into the kernel in case it deserves slow path processing. When the kernel responds, I read the packets back from the tap and send it out via the packet socket. I use iptable rules to drop the packets at the entry from the interface so that they are not processed by kernel directly (because I read them via packet socket and then inject into the kernel via the tap interface) iptables -A INPUT -i eth0 -j DROP iptables -A FORWARD -i eth0 -j DROP
Now the above mechanism works very well for me except when the slow path encounters fragmented IP packets. When I inject the fragmented IP packets into the kernel via the tap interface, the kernel does not.respond (eg. for a ping bigger than mtu size) I have checked with tcpdump on tap that I have injected all the fragments into the kernel. Strangely enough, the same usecase works if I put the delays (usleep) at two places in my application -- 1. Just before writing the packet to tap (injection into the kernel) 2. Just after reading the kernel response from the tap and just before sending the packet out using the packet socket. The delays work for me but is clearly not good for the performance of the slow path. And more importantly, I was looking for a fundamental reason regarding why it works with delays and why not without it. The issue is reproducible with a big ping (3.11.10-301.fc20.x86_64) Regards -Prashant -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
